Transit time modulator



Dec. 23, 1958 N. w. LEDBETTER TRANSIT TIME MODULATOR Filed July :50, 1957 la-l INVENTOR. N05; (1 Amen-7:2

BY I WM Ra Irranwtr! United States Patent TRANSIT TIME MODULATOR Noel W.Ledbetter, Los Angeles, Calif., assignor to Lenkurt Electric Co., Inc.,San Carlos, Calif a comeration of Delaware Application July 30, 1957,Serial No. 675,103 2 Claims. (Cl. 31382) This invention relates toapparatus for modulating the phase or frequency of an electric signal byvarying the transit time or transmission delay of the signal through anelectron tube.

Briefly stated, according to certain aspects of this invention, animproved transit time modulator comprises an electron tube having meansfor forming an electron beam, means for varying the intensity of theelectron beam responsive to an input carrier signal, means for producinga variable transverse deflection of the electron beam responsive to aninput modulating signal, and output means for producing an outputelectric signal responsive to the variations in the intensity of theelectron beam. The output means is disposed in slanting relation to thebeam so that as the beam is deflected transversely responsive to themodulating signal the effective length of the beam and the transit timeof the signal passing through the tube are varied. Preferably the outputmeans includes an electrode surface that lies along a segment ofaparabola having its axis disposed perpendicular to the middle of themean path followed by the electron beam between the deflection means andthe output means. Also, means are provided for maintaining asubstantially field-free drift space in which the electron beam moves atsubstantially constant velocity between the deflection means and outputmeans.

The foregoing and other aspects of this invention may be betterunderstood from the following illustrative description and theaccompanying drawings. The scope of the invention is pointed out in theappended claims.

In the drawings:

Fig. 1 is a schematic diagram of an improved transit time modulator;

Fig. 2 is a longitudinal section showing one construction of a driftspace shield and collector electrode'for the improved transit timemodulator;

Fig. 3 is a transverse section taken alongline 3-3 of Fig. 2; and

Fig. 4 is a fragmentary perspective view showing another construction ofthe drift space shield and collector electrode.

Referring to Fig. 1, an improved transit time modulator employs a novelelectron tube contained in an evacuated envelope 1. A conventionalelectron ,gun comprising a cathode 2 and accelerating electrode 3 isoperable to produce within envelope 1 an electron beam, represented inthe drawing by broken line 4. The electron gun also includes aconventional control electrode 5 for varying the intensity of theelectron beam responsive to an input electric signal supplied betweenthe control electrode and the cathode as hereinafter explained.

Conventional electrostatic deflection electrodes 6 and 7 are providedfor producing a variable transverse deflection of beam 4 responsive toanother inputelectric signal. Thus, the electron beam can be deflectedthrough a range of angular directions. In the drawing, broken line 4reppresents a mean or undeflected path of the electron beam while brokenlines 4 and 4" represent the extreme up- Patented Dec. 23, 1958 Ward anddownward deflections of the beam, respectively.

A collector electrode 8 is disposed in slanting relation to the electronbeam, as shown, so that deflection of the beam by the deflectionelectrodes moves the electron beam along the slant of the collectorelectrode and varies the length of the electron path between the controlelectrode 5 and the collector electrode 8.

A substantially field-free drift space, through which the electrons moveat substantially constant velocity, is provided between the deflectionelectrodes and the collector electrode. This field-free drift space maybe defined, for example, by an electron permeable grid 9 disposed justin front of collector electrode 8, as shown, and a lateral shield 10.Grid 9 serves two principal purposes. First, it prevents variations inthe potential of collector electrode 8 from having a substantial effecton the transit time of electrons traveling between the deflectionelectrodes and the collector electrode. Second, it improves the frequency response of the tube by providing an electrostatic shield betweenthe collector electrode and the electrons approaching the collectorelectrode until such approaching electrons are closely adjacent to thecollector electrode.

Preferably, collector-electrode 8 and grid 9 which are closely adjacentto each other lie substantially along a segment of a parabola having itsaxis disposed perpendicular to the mean or undeflected path 4 of theelectron beam between the deflection electrodes and the collectorelectrode.

Accelerating electrode 3, grid 9, and transverse shield 10 areelectrically connected together and maintained at the same constantelectric potential. Deflection electrodes 6 and 7 are maintained at thesame average potential as accelerating electrode 3. Consequently, exceptfor the transverse deflecting potential applied between deflectionelectrodes 6 and 7 as is hereinafter explained, the space betweenaccelerating electrode 3 and grid 9 i substantially field-free andelectrons travel through this space at a constant horizontal velocity.

Balanced modulating potentials, /2e and /2e relative to cathode 3, areapplied to deflection electrodes 6 and 7, respectively, by any source 11of an input modulating electric signal. Consequently, a modulatingvoltage e is applied between deflecting electrodes 6 and 7 whichdeflects the electron beam transversely through the angle between brokenlines 4' and 4". Cathode 2 is maintained at a constant negativepotential relative to accelerating electrode 3 by a conventional voltagesupply 12.

The voltage between electrode 3 and cathode 2 determines a constanthorizontal component of electron velocity in the electron beam betweenelectrode 3 and grid 9. In addition, the electrons have a verticalcomponent of velocity that is proportional to the deflecting voltagebetween deflecting electrodes 5 and 7 at the instant when the electronspass between the two deflecting electrodes.

Control electrode 5 is connected through a resistor 13 to a negativebias voltage supply 14, which biases control electrode 5 to a negativepotential relative to cathode 2. In addition, an alternating carriervoltage e provided by any suitable source 15, is supplied to controlelectrode 5 through a coupling capacitor 16. Thus the input signal esuperimposed on the bias voltage provided by supply 14, is appliedbetween control electrode 5 and cathode 2 for modulating the intensityof the electron beam within envelope 1. In other words, the intensity ofthe electron beam is varied as a function of the input carrier signal aand the transverse deflection of the electron beam is varied as'afunction of the input modulating signal e Collector electrode 8 isconnected through a load resister 17 to a voltage supply 18 whichpreferably maintains collector electrode '8 at a more positive potentialthan grid 9. Thus there is a voltage gradient between collectorelectrode 8 and grid 9 which quickly draws electrons into the collectorelectrode as they pass through gird 9. This produces an electric currentflowing from supply 18 through load resistor 17 into collector electrode8. Variations in this current produce variations in the voltage dropacross load resistor 17 which in turn produces an alternating componentof output voltage e which is transmitted through coupling capacitor 19to any desired utilization circuit.

For a better understanding of the apparatus, assume first that themodulating voltage e has zero amplitude. Then the electron beam withinenvelope 1 is undeflected and travels in a straight line between cathode2 and grid 9 along the mean path represented by broken line 4. As theelectrons pass through grid 9 they are quickly drawn into collectorelectrode 8 and produce a flow of electric current into the collectorelectrode through load resistor 17.

The intensity of the electron beam is modulated by the input carriersignal e At each positive peak of the input carrier signal a relativelydense bunch of electrons passes through control electrode of theelectron gun. These electron bunches travel along the electron beambetween electrode 3 and grid 9 at a velocity proportional to the squareroot of the voltage provided by supply 12. As each bunch of electronstravels down the beam it is electrically shielded from the collectorelectrode 8 until approximately the instant that the electrons passthrough grid 9.

As each bunch of electron flows across the space between grid 9 andcollector electrode 8 current flows into the collector electrode due tothe positive charges induced thereon by the electrons flowing toward thecollector electrode, which charges are neutralized as the electronsenter electrode 8. Therefore, provided that the transit time ofelectrons between grid 9 and electrode 8 is short compared to the periodof the input carrier signal e each bunch of electrons in the electronbeam causes a momen tary increase in the current flowing into collector8 and the voltage drop across load resistor 17 contains an alternatingcomponent 2 that is substantially identical to the input carrier signal:2 delayed by the transit time of the electrons between controlelectrode 5 and collector electrode 8.

It will be noted that the frequency response of the tube is limited bythe transit time of electrons traveling between grid 9 and controlelectrode 8. Therefore, a high frequency response can be obtained byproviding a close spacing between collector electrode 8 and grid 9 andby providing a sufllcient voltage at the collector electrode to draw theelectrons quickly into the collector electrode as they pass through grid9.

Now assume that there is supplied between deflection electrodes 6 and 7a modulating voltage e of suflicient r amplitude to deflect the electronbeam repetitively back and forth between the paths represented by brokenlines 4 and 4". As the electron beam is deflected back and forth thelength of the beam varies considerably because of the slanting relationof collector electrode 8 and grid 9 to the beam. Furthermore, since theelectrons travel between the accelerating electrode 3 and grid 9 with asubstantially constant horizontal velocity the transit time of eachelectron bunch varies as a function of the length of the path which thatbunch follows and therefore varies as a function of the instantaneousvalue of the modulating voltage e at the instant when that bunch ofelectrons passes between the deflection electrodes 6 and 7.

Assume that the instantaneous amplitude of the modulating voltage e ischanging in a positive direction so that the electron beam is beingdeflected from the path 4" toward the path 4. It is evident that eachbunch of electrons must travel a somewhat longer path than the precedingbunch and therefore has a somewhat longer transit time between controlelectrode 5 and collector electrode 8. Consequently, successive cyclesof the output voltage :2 are spaced somewhat further apart thansuccessive cycles of the input voltage 2 Conversely, when theinstantaneous values of the modulating voltage e are becoming morenegative the beam is being deflected from path 4 to path 4" andsuccessive cycles of the output voltage e are more closely spaced thansuccessive cycles of the input voltage e Thus, a type of modulation isproduced which is somewhat similar to phase and frequency modulation.The technical name for this type of modulation is transit-timemodulation and its characteristics are already known to those skilled inthe art.

By making electrode 8 and grid 9 lie substantially along a segment of aparabola, as hereinbefore explained, the transit time is made to vary asa linear function of the instantaneous amplitude of the modulatingvoltage c This is desirable to avoid distortion.

By proper configuration of the shielding electrode 10 the use of aphysically present grid 9 of wires or the like can be avoided. This isillustrated in Figs. 2 and 3 which show a simple construction that maybe employed for collector electrode 8 and drift-space shield 10. InFigs. 2 and 3 the collector electrode is identified by the referencenumeral 8 and the shield is identified by the reference numeral 10'.These two parts may be made from a single strip of metal folded to aU-shaped cross-section as is best shown in Fig. 3 and cut into twoelectrically separate parts 8' and 10 separated by a gap 20, as shown.The gap 20 is cut along a segment of a parabola so that the right edgeof part 10 corresponds in shape and position to grid 9 of Fig. 1 whilethe left end of part 8 corresponds in shape and position to electrode 8of Fig. 1.

The two flat sides of the U-shaped strip are just far enough apart forthe electron beam to pass between them. Since these two flat sides areclose together and are electrically conductive external electric fieldscan penetrate only a short distance into the space between the two flatsides of the metal strip. Consequently, substantially all of the spacebetween the two flat sides of part 10' is maintained at a substantiallyconstant electric potential and electrons travel through this space at asubstantially constant velocity. The principal effect of electric fieldspenetrating a short distance into the space between the two flat sidesof part 10 is that the effective electrical width of gap 20 is somewhatwider than its actual physical width. When the electrons reach gap 20they are quickly drawn across the gap by the voltage gradient providedby maintaining part 8 positive with respect to part 10'.

As the electrons cross gap 20 they induce positive charges on collectorelectrode 8'. Therefore, electrons may be considered to have beencollected by electrode 8' as soon as they have crossed gap 20 andwhether the electrons immediately strike electrode 8 or travel for somedistance within a space that is substantially enclosed by the collectorelectrode is of little importance. Therefore, the frequency response ofthe tube depends only upon the transit time of the electrons across theelectrical width of gap 20 which comprises the physical width of the gapplus some additional width due to the slight penetration of electricfields into the space between the two parallel flat sides of parts 10'and 8'.

To prevent the escape of any electrons out the back end of collectorelectrode 8 this end of the U-shaped metal strip can be closed by aninsert 21, as shown, or by simply pressing the two sides of the U-shapedstructure together at this end.

To increase the frequency response or to permit a wider space for theelectron beam between the two flat sides of the U-shaped metal strip, orboth, the alternative construction illustrated in Fig. 4 may be used. Inthis construction collector electrode 8" has substantially the sameexterior shape as collector electrode 8' of Fig. 2 but in theconstruction illustrated in Fig. 4 the collector electrode is made fromone solid piece of metal and has no hollow interior space. 7 Therefore,the electrons strike the collector electrode immediately upon crossingthe gap between the parts and 8" and there can be no widening of the gapdue to the penetration of electric fields into the collector electrode.

The shield 10" may be substantially identical to part 10' shown in Figs.2 and 3 but, if desired, the two flat sides of part 10" may be somewhatfurther apart than the two flat sides of part 10 to provide more spacefor the passage of the electron beam. A plurality of parallel wires arewelded or otherwise attached transversely across the right end of part10" extending between the two flat parallel sides and forming a grid 9"close to but slightly separated from collector electrode 8". Grid 9"corresponds to grid 9 of Fig. 1. The grid reduces the penetration ofelectric fields into the space between the two flat parallel sides ofpart 10" and thus makes the effective electrical width of the gapbetween parts 10" and 8" more nearly equal to the physical width of thegap. This increases the frequency response of the tube and makespossible the use of the apparatus with higher carrier frequencies.

It is evident that the transit time modulators herein described can beused for a considerable variety of purposes. For example, if the size ofthe tube and the magnitude of the voltage supplied by supply 12 are madesuch that the transit time along path 4 differs from the transit timealong 4 by one period of the carrier signal e then a full 360 degrees ofphase modulation is easily obtained responsive to a modulating voltage eof moderate amplitude. Furthermore, by properly shaping the output means(essentially the gap between grid 9 and collector electrode 8 or itsequivalent) into a parabolic segment as herein disclosed theinstantaneous phase shift can be made linearly proportional to theinstantaneous amplitude of the modulating voltage, thereby providingmodulation of exceptionally low distortion which is difiicult to obtainby other methods. If the modulating voltage e has a sawtooth waveformproviding successive 360 degree phase shifts separated by relativelyshort flyback intervals low-distortion frequency translation can beachieved without resort to the heterodyne principle.

Numerous variations in construction are of course possible. For example,the electrostatic deflection electrodes 6 and 7 can be replaced withmagnetic deflection means in a manner similar to the common use ofmagnetic deflection in television picture tubes. Where magneticdeflection is employed a slight reshaping of the collector electrode 8and grid 9 may be desirable due to the wellknown diflferences betweenthe electron ballistics of electrostatic and magnetic deflection.However, such corrections will generally not be large and in practicalcases will often be negligible.

It should be understood that this invention in its broader aspects isnot limited to specific examples herein illustrated and described andthat the following claims are intended to cover all changes andmodifications within the true spirit and scope of the invention.

What is claimed is:

1. A transit time modulator comprising an evacuated envelope, anelectron gun for producing an electron beam within said envelope, saidgun including a cathode, a control electrode, and an acceleratingelectrode, means for supplying a first variable voltage between saidcontrol electrode and said cathode for varying the intensity of saidbeam, a collector electrode disposed in slanting relation to said beam,deflection means for producing a variable transverse deflection of saidbeam to move said beam along the slant of said collector electrode, andshielding means for maintaining a substantially field-free space throughwhich the electrons of said beam travel at substantially constantvelocity between said deflecting means and said collector electrode,said shielding means comprising a folded metal strip having asubstantially U- shaped cross section.

2. An electron tube comprising within an evacuated envelope thecombination of an electron gun operable to produce an intensitymodulated electron beam, a conductive shield having two parallel flatsides disposed on opposite sides of said beam, and a collector electrodefor receiving said beam, said colector electrode and said shieldingmeans being separated by a gap having the shape of a segment of aparabola, said gap being disposed in slanting relation to said beam.

References Cited in the file of this patent UNITED STATES PATENTS2,071,382 Balsley Feb. 23, 1937 2,290,587 Goldstine July 21, 19422,401,740 Kilgore June 11, 1946 2,519,443 Diemer Aug. 22, 1950

